Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936991AbXHJOMu (ORCPT ); Fri, 10 Aug 2007 10:12:50 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1764719AbXHJOMm (ORCPT ); Fri, 10 Aug 2007 10:12:42 -0400 Received: from x346.tv-sign.ru ([89.108.83.215]:50710 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763686AbXHJOMl (ORCPT ); Fri, 10 Aug 2007 10:12:41 -0400 Date: Fri, 10 Aug 2007 18:15:01 +0400 From: Oleg Nesterov To: "Serge E. Hallyn" Cc: Pavel Emelyanov , Andrew Morton , Linux Kernel Mailing List , Linux Containers , "Eric W. Biederman" , "Paul E. McKenney" , devel@openvz.org Subject: Re: [PATCH] Make access to task's nsproxy liter Message-ID: <20070810141501.GB74@tv-sign.ru> References: <46BAE3A3.7030608@openvz.org> <20070810134003.GA22368@sergelap.austin.ibm.com> <20070810140545.GA74@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070810140545.GA74@tv-sign.ru> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2189 Lines: 57 On 08/10, Oleg Nesterov wrote: > > On 08/10, Serge E. Hallyn wrote: > > > > Quoting Pavel Emelyanov (xemul@openvz.org): > > > +/* > > > + * the namespaces access rules are: > > > + * > > > + * 1. only current task is allowed to change tsk->nsproxy pointer or > > > + * any pointer on the nsproxy itself > > > + * > > > + * 2. when accessing (i.e. reading) current task's namespaces - no > > > + * precautions should be taken - just dereference the pointers > > > + * > > > + * 3. the access to other task namespaces is performed like this > > > + * rcu_read_lock(); > > > + * nsproxy = task_nsproxy(tsk); > > > + * if (nsproxy != NULL) { > > > + * / * > > > + * * work with the namespaces here > > > + * * e.g. get the reference on one of them > > > + * * / > > > + * } / * > > > + * * NULL task_nsproxy() means that this task is > > > + * * almost dead (zombie) > > > + * * / > > > + * rcu_read_unlock(); > > > > And lastly, I guess that the caller to switch_task_namespaces() has > > to ensure that new_nsproxy either (1) is the init namespace, (2) is a > > brand-new namespace to which noone else has a reference, or (3) the > > caller has to hold a reference to the new_nsproxy across the call to > > switch_task_namespaces(). > > > > As it happens the current calls fit (1) or (2). Again if we happen to > > jump into the game of switching a task into another task's nsproxy, > > we'll need to be mindful of (3) so that new_nsproxy can't be tossed into > > the bin between > > > > if (new) > > get_nsproxy(new); > > 4) Unless tsk == current, get_task_namespaces(tsk) and get_nsproxy(tsk) > are racy even if done under rcu_read_lock(). (sorry for noise, but I'm afraid I was not clear again...) This looks OK, we don't do get_nsproxy(not_a_current), but perhaps it is not immediately obvious that we shouldn't. Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/